Overload warning device for fork lift trucks



Jan. 13, 1959 F/ZEJHARVEY, 2,368,919

OVERLOAD WARNING DEVICE FOR FORK LIFT TRUCKS Filqd April. 15, 1957 2 Sheet s-Sheet 1 Int GINO! FRANK GAVIN HARVEY Attorney Jan. 13, 1959 F. G. HARVEY 6 OVERLOAD WARNING DEVICE FOR FORK LIFT TRUCKS Filed April 15, 1957 2 Sheet S-Sheet 2 4 30 55 Inventor kc L fl-"k k' FRANK GAVIN HARVEY -lwhim? Attorney United States Patented Jan. i3, 195% OVERLOAD WARNING DEVICE FOR FORK LIFT TRUCKS Frank Gavin Harvey, Montreal, Quebec, Ca

to Canadian Lift Truck Company L... fed

The present invention relates to overload warning devices for vehicles and more specifically to an overload warning device for cantilever-type lift trucks.

It is a fundamental property of the cantilever-type of fork lift truck that the load on the forks may not exceed the counterbalancing effect of the weight of the truck itself back of the front axle (drive axle). If this limitation is exceeded, the truck will tip over forwards, and this possibility is dangerous. It should be noted that the overbalancing effect of a load on the forks depends not only on its weight but on its size, because if for example the weight remains constant but the size of the load increases, the center of the gravity of the load moves further from the drive axle center which is the fulcrum, and the load thus exerts greater leverage.

Fork trucks are therefore usually rated by manufacturers to have a certain maximum capacity at a specified load center, which is defined as the distance from the center of gravity of the load to the face of the fork carriage. A commonly used rating is 4,000 lbs. at 24 inch load center which means the truck will lift, with the proper margin of safety, a load weighing 4,000 lbs. and measuring 48 long on the longitudinal axis of the truck.

In practice, widely differing loads are often handled by lift truck operators who have no means of determining the weight of each. The usual solution is to pick up and carry whatever load the truck will carry without lifting its rear end off the ground. Under these circumstances a load may be picked up which leaves no margin'of safety at all, so that if the operator then tilts his mast forward as he may require to do when stacking, or is obliged to brake sharply when travelling forward, the truck will tilt and fall with resulting damage by the hoist mechanism of a lift truck, as distinct from the weight it can carry without overturning, is related to the hydraulic pressure in the line supplying the hydraulic lift cylinder, and this principle has been used calibrate a hydraulic pressure gauge so that it reads as a scale, indicating the weight on the forks within broad limits.

It has been contemplated that the same principle might be utilized to operate a warning device when a given weight on the forks was exceeded. However, the weight of a load has no bearing on its actual size and as discussed above the overbalancing tendency of the load does not depend on weight alone but is a function of its weight and size.

With this in mind the present invention aims to provide a practical solution to the problem by furnishing an overload warning device which is actuated by the actual displacement or raising of the rear end of the truck as may be caused by an overload, with respect to size and/or weight, on the lifting forks. More specifically, the rear axle (steering axle) of a fork lift truck is usually mounted on semi-elliptic springs, which determine the position of the axle in relation to the truck chassis. When there is no load on the forks the full weight of the rear end of the truck rests on the steering axle springs, which then attain their maximum static deflection. As the load on the forks is increased, the load reaction takes weight off the rear axle until at the moment when the rear end leaves the ground, the rear axle is practically hanging from the springs. The flexibility of the springs will thus allow a variation in the vertical distance from axle to chassis, proportional to any change in the load on the forks, and this variation will be a true function of the overbalancing effect of the load, rather than of its weight only.

Accordingly, the invention is an overload warning apparatus for cantilever lift trucks of the type having a body frame, a load lifting fork on the front end of the body frame, a front axle mounted on the body frame adjacent the lifting fork and wheels mounted on the front axle, a rear axle resiliently suspended from the frame and wheels mounted on the rear axle with the front axle serving as a fulcrum when a load is applied to the lifting fork; the apparatus of the invention consists of a switch assembly mounted on the rear axle and circuit means between the switch assembly and an electrical warning device mounted on the frame. A lever arm is supported for pivotal movement about a fixed axis on the rear axle with one end of the lever arm adjustably contactin the body frame with the other free end disposed in spaced alignment with and resiliently biased towards the contact member of the switch assembly. iiith this arrangement, raising of the rear portion of the frame about the front axle fulcrum by an overload applied to the lifting fork is adapted to bring the free lever end into contact with the switch contact memher and thus activate the warning device.

Having thus generally described the nature of the invention particular reference will be made to the accompanying drawings showing a preferred embodiment thereof, and in which:

Figure 1 is a somewhat diagrammatic view in side elevation of a lift truck embodying an overload warning apparatus in accordance with the invention with the rear axle assembly shown partially in section and the rear wheels indicated in dotted lines with the lifting fork unloaded.

Figure 2 is a partial side elevational view of the front and rear portions of the construction shown in Figure l with the lifting fork loaded causing deflection of the rear axle assembly relative to the body frame.

Figure 3 is a view in perspective elevation showing a typical lift truck rear or steering axle with the apparatus of the invention mounted thereon and a portion of the attaching bracket to the body frame with the main controlling lever of the apparatus in adjustable contact therewith.

Figure fis a plan view of the apparatus of the invention with the ca in section an showing a portion of the body frame attaching bracket.

Figure 5 is a side elevation of the apparatus shown in Figure 4.

Figure 6 is a bottom plan view of the lift truck con- Struction shown in Figure l to illustrate the relative position of the apparatus of the invention.

Figure 7 is a diagrammatic showing of the wiring circuit for the apparatus of the invention.

With particular reference to Figures 1 and 2, a cantilever-type lift truck is shown which includes a main frame or chassis and lift fork assembly 12 mounted on the ascents front of the frame lltl. Since the construction and operation of the lift truck assembly 12 are well known, and do not form a part of the invention, it is not thought necessary to illustrate or describe this in more detail. A front axle assembly 1.4 is mounted on the frame 1d and front wheels 16 are mounted on this assembly. In the type of lift true. shown the motive power is applied through the axle assembly 14 to the wheels 16.

A rear or steering axle 18 is supported on the frame Ill by semi-elliptic springs Zil which accordingly determine the vertical position of the axle ill relative to the frame 1b. Wheels 21 are mounted on the axle if in the usual manner. When there is no load on the lifting fork assembly 12, as shown in Figure l, the full weight of the rear of the lift truck rests on the springs 2ft. As a lo' is applied to the lift fork assembly E2 the front axle serves as a fulcrum and the load reaction takes weight off or raises the rear portion of the frame until the rear axle is practically hanging from the springs 2?, thus varying the vertical distance between the rear axle 18 and frame.

Since this variation in vertical distance between the axle and frame is proportional to any change of load on the lifting forks and is a true variation of the over-balancing effect of the load, rather than of its weight only, it is utilized to operate the warning apparatus of the invention in the following manner.

A switch assembly 3% is mounted in a suitable casing or housing 32 which is secured at the center of the rear axle 13. The switch assembly 33 includes a micro-switch 33 and a rocking beam or lever arm 3ais mounted for pivotal movement on the casing 32 with one end 35 in alignment with the switch 33. The other end 36 or" the lever arm adjustably contacts a bracket ill attached to the frame 163 by an adjustable screw 42. in order that the movement of the lever arm 34 be as sensitive as possible, while not being affected by the normal movements of the truck, it is provided with a knife-edge pivot blade 44 engaged in a slotted bar 46 mounted on the side of the casing 32. The end 35 of the lever arm 34 is guided in its arcuate movement by a guide rod l3 passing through an opening 50 provided in the lever end. A coil spring 52 extends between the lever arm 34 and the casing 32 adjacent the end 35 so as to normally urge it towards the microswitch 33.

As the axle 18 moves away from the frame fit under the influence of a load on the fork assembly 12, the end of the lever arm 34 is drawn down by the spring 523 until it operates the micro-switch 33, thus operating a horn 54 or other warning device. By means of the adjusting screw 42 the device can be adjusted to operate whenever say, for example, 85% of the overtipping load is applied to the truck, regardless of the actual dimensions or weight of the object applying the load.

As will be appreciated, a simple circuit connection between the warning apparatus of the invention and the horn 54 would likely result in the warning operating from time to time as the truck moved about under load, due to the normal bounce and deflection of the rear springs Zll.

To avoid this, and as shown diagrammatically in Figure 7, the circuit between the contact switch 33 and the horn 54 is preferably wired in series with electrical contact members A, 33, disposed on or near the usual hydraulic lift and tilt controls respectively for the lifting fork assembly 12 so that they are closed, to complete the circuit, only when these controls are operated to lift or tilt the lifting fork. As shown, a horn button or switch C is located in the circuit so that it will operate as usual. When wired in this manner the warning apparatus of the invention will act to operate the horn 54 only if an attempt is made to lift an overload or to tilt it forward.

I claim:

1. An overload warning apparatus for cantilever-type lift trucks having a body frame, a load lifting fork on the front end of said body frarrife, a front axle mounted on said body frame adjacent said lifting fork and wheels mounted on said front axle, a rear axle resiliently sus pended from said frame and wheels mounted on said rear axle, said front axle serving as a fulcrum when a load is applied to said lifting fork; said apparatus comprising a switch contact member mounted on said rear axle, circuit means between said switch contact member and an electrical warning device mounted on said frame, a lever arm separate from said rear axle suspension and supported for pivotal movement about a fixed axis on said rear axle at a point spaced from said switch contact member and having one end spaced from said pivot point and adjustably contacting said body frame with the other end also spaced from said pivot point and guided in aligned relationship with said switch contact member, means normally urging said aligned lever end towards said switch contact member, whereby raising of the rear portion of said frame about said front-axle fulcrum by a load applied to said lifting fork is adapted to cause said aligned lever arm end to contact with said switch contact member activating said warning device.

2. An overload warning device, as claimed in claim 1, wherein the means normally urging the end of said lever arm aligned with said switch contact member towards said member is a coil spring extending between said lever arm end and said rear axle.

3. An overload warning device for cantilever-type lift trucks having a body frame, a load lifting fork on the front end of said body frame, a front axle mounted. on said body frame adjacent said lifting fork and wheels mounted on said front axle, a rear axle mounted on spring members suspended from said frame and wheels mounted on said rear axle, said front axle serving as a fulcrum when a load is applied to said lifting fork; said apparatus comprising a casing including a side wall opening mounted centrally of said rear axle, a switch assembly mounted in said casing and including a switch contact member, circuit means between said switch contact member and an electrical warning device mounted on said truck, a lever arm separate from said rear axle suspension and mounted for pivotal movement about a fixed axis on said casing with both ends free and extending outwardly from said pivotal axis, one end of said lever arm being disposed within said casing and guided in aligned relationship with said switch contact member with the other end extending from said casing opening to an adjustable contact with said body frame in front of said rear axle, and means within said casing normally guidin and resiliently urging said inner lever end towards and into contact with said switch contact member when the vertical distance between said rear axle and frame is increased beyond a predetermined distance by an overload applied to said load lifting fork.

4. An overload farning device, as claimed in claim 3, wherein said means for guiding said lever one end includes an arcuate guide member extending upwardly from said switch assembly, said lever end having an opening adapted to slidably receive said arcuate guide member, and a tension spring connected between said lever and and said casin".

5. An overload warning device as claimed in claim 4, wherein the lever arm end disposed exteriorly of said casing is provided with an adjustable screw and there is a bracket mounted on said body frame in alignment with said adjustable screw.

References Cited in the file of this patent UNITED STATES PATENT 8 2,704,132 Marco Mar. 15, 1955 

